1. Field of the Invention
The present invention relates to a method for producing an aqueous dispersion containing a complex of poly(3,4-dialkoxythiophene) and a polyanion, and relates to an aqueous dispersion obtained by the method.
2. Description of the Related Art
Conductive transparent films are employed for coating transparent electrodes of liquid crystal displays, electroluminescence displays, plasma displays, electrochromic displays, solar cells, touch panels and the like, and for coating substrates such as those made of electromagnetic shielding material. The most widely employed conductive transparent film is a vapor deposited film made of indium-doped tin oxide (i.e., ITO). However, the formation of ITO film by a vapor deposition process has problems such as the necessity of a high temperature for film formation and high cost for film formation. ITO film can be formed by a coating method. However, the film formation by this method requires a high temperature, the conductivity of the film depends on the degree of dispersion of ITO, and the haze value of the film is not always low. Furthermore, for an inorganic oxide film such as an ITO film, cracks tend to be generated by the bending of the substrate, so that the conductivity may be reduced.
On the other hand, a conductive transparent film made of conductive polymer that can be prepared at a low temperature and a low cost has been proposed as a conductive transparent film made of an organic material. As for the conductive polymer that can be used for such a film, Japanese Patent Publication No. 2636968 discloses a method for producing a complex of poly(3,4-dialkoxythiophene) and a polyanion. This complex has good water dispersibility. A thin film produced by applying a coating composition containing an aqueous dispersion containing the complex to a substrate has a sufficient antistatic function, but insufficient transparency and conductivity.
Japanese Laid-Open Patent Publication No. 8-48858 describes that a thin film produced by applying a coating composition to a substrate has an improved conductivity, wherein the coating composition is obtained by adding a compound that is selected from the group consisting of a compound having two or more hydroxyl groups, a compound having an amide group, and a compound having a lactam group to an aqueous dispersion containing a complex of poly(3,4-dialkoxythiophene) and a polyanion described in Japanese Patent Publication No. 2636968. Japanese Laid-Open Patent Publication No. 2000-153229 describes that a thin film produced by applying a coating composition containing a non-proton compound having a dielectric constant of ∈≧15 to a substrate and drying the resultant substrate at a temperature less than 100° C. has an improved conductivity.
All the coating compositions described in these publications have improved properties by adding a specific compound to the aqueous dispersion containing a complex of poly(3,4-dialkoxythiophene) and a polyanion described in Japanese Patent Publication No. 2636968, and the conductivities of the resultant films are comparatively improved. However, the aqueous dispersion containing the complex (i.e., a conductive polymer) is the same, so that the transparency and the conductivity of the resultant film obtained from the aqueous dispersion are not necessarily sufficient.
In Japanese Laid-Open Patent Publication No. 2004-59666 discloses that an aqueous dispersion containing a complex that can form a thin film having excellent transparency and conductivity can be obtained by using peroxodisulfuric acid as an oxidizing agent, when polymerizing 3,4-dialkoxythiophene in the presence of a polyanion or adding an acid to lower the pH at the time of polymerization. With this method, a thin film having relatively good transparency and conductivity can be formed, but there is a demand for development of a material that can be formed into a thin film having even better transparency and conductivity and a method for producing such a thin film.